As a method of molding a pipe formed of a thermosetting resin, extrusion molding and blow molding have been used conventionally.
Although a pipe formed by the extrusion molding is essentially a straight pipe, the pipe is formed of a soft material, or the pipe has a corrugated design, for example, whereby the pipe can obtain flexibility. However, in the extrusion molding method, a pipe material itself is rigid, and thus a bent pipe having smooth inner and outer surfaces cannot be obtained. Moreover, a flange portion and an attachment portion cannot be integrally molded to a pipe, and a molded body portion having a coupling function cannot be integrally molded to a pipe end portion.
In the blow molding method, although a three-dimensional bent pipe can be molded by the recent development of a mold tilting mechanism and a multipurpose blow molding machine having a mold swinging apparatus, the thickness accuracy, the surface finishing accuracy, the dimensional accuracy, component integrating function, and so on do not attain a satisfactory level compared to injection molding. Further, the blow molding method has many problems that the material to be used is limited in terms of characteristics such as the fluidity and the melt viscoelasticity of the materials, and especially engineering plastics are strongly subjected to the above limitations.
As a method of forming a hollow body by injection molding, there has been known a gas assist injection molding method (Patent Documents 1 and 2). However, although a hollow pipe portion can be molded by this method, a uniform inner diameter cannot be secured. Although there has been known a water assist injection molding (WIT) method of forming the hollow pipe portion with the use of water as a fluid instead of gas (Non Patent Document 1), this method has a problem in the uniformity of a pipe inner diameter, the uniformity of pipe thickness, the smoothness of a pipe inner surface, and so on, and thus there is a limit to the expansion of pipe molding by the injection molding.
As a molding method that solves the above problems, a method using a floating core has been known (Patent Documents 3 and 4). According to the method described in the Patent Documents 3 and 4, the uniformity of the inner diameter and the thickness of a pipe to be manufactured is far excellent compared to the method described in the Patent Documents 1 and 2. However, in order to apply the method to a pipe required to have high strength and high durability, a higher level of uniformity in the pipe inner diameter and the pipe thickness and the smoothness of the pipe inner surface are required, and it is a fact that there is a limit to the increasing use of this method.